1. Field of the Invention
The present invention relates to variable priority devices, and more particularly to a variable priority device employed in a variety of construction machines such as excavators.
2. Description of the Prior Art
Where at least two actuators are operated in a combined manner by oil delivered from a single pump, a "priority" is established to control the actuators such that one of the actuators is supplied with a larger amount of oil than the other actuator. For example, excavators have a priority of the swing actuator over the arm actuator and a priority of the boom over the bucket. The reason why such priorities are given is because in most cases, the amount of oil required for a swing operation is larger than the amount of oil required for an arm operation, and the amount of oil required for a boom operation is larger than the amount of oil required for a bucket operation. As oil is supplied in different amounts depending on the kind of operation in accordance with the priorities, it is possible to prevent an unnecessary loss of pressure and achieve a smooth operation.
In order to provide such a priority function, there have been used stroke limiters, fixed orifices and variable orifices.
Referring to FIG. 1, there is shown a conventional stroke limiter. The stroke limiter, which is denoted by the reference numeral 101, is installed in a control valve A equipped in an actuator which is associated with the stroke limiter. The stroke limiter 101 serves to limit the stroke of the spool of control valve A within a desired range, thereby preventing the fluid supply passage 105.
A conventional fixed orifice is illustrated in FIG. 2. As shown in FIG. 2, the fixed orifice denoted by the reference numeral 201 is disposed in a parallel oil passage 203 to always limit the amount of oil supplied to a control valve A of the actuator associated therewith.
However, the above-mentioned conventional devices have a problem that the oil passage or line is always limited on the oil amount passing therethrough, irrespective of whether the actuator associated operates alone or in combination with the other actuator. Where the associated actuator operates alone, the limitation on the oil amount results in various problems such as an unnecessary loss of pressure and a decrease in the operating speed of the actuator.
Referring to FIG. 3, there is illustrated a variable orifice. As shown in FIG. 3, the variable orifice denoted by the reference numeral 301 is installed in a parallel fluid line 303. The variable orifice 301 is switched between its orifice state and its orifice release state in response to a pilot pressure Pi for moving the spool of a control valve 302. The variable orifice 301 is initially set to be at the orifice release state by a spring 305 when no pilot pressure is exerted. When the control valve 304 operates alone, the variable orifice 301 is maintained at its orifice release state because no pilot pressure is exerted thereon. At this state, a sufficient amount of fluid is normally supplied to the control valve 304. Only when the control valve 302 operates, the variable orifice 301 is switched to its orifice state by the pilot pressure Pi exerted thereon against the resilience of the spring 305, thereby performing its priority function. That is, the variable orifice 301 decreases the amount of fluid supplied to the control valve 304 and correspondingly increases the control valve 302 by the decreased fluid amount.
However, such a variable orifice involves a problem that an unnecessary loss of pressure occurs at the fluid line associated with the control valve 304 due to the orifice function when the load of the actuator associated with the control valve 304 is rather larger than that of the actuator associated with the control valve 302, nevertheless it is unnecessary in this case to provide the orifice function for establishing a desired priority. Where the actuator associated with the control valve 304 and the actuator associated with the control valve 302 are an arm cylinder and a swing motor, respectively, the load applied to the arm cylinder may be larger than that applied to the swing motor. Even in this case, the conventional variable orifice limits the amount of fluid supplied to the arm cylinder because it is constructed to always limit the amount of fluid supplied to the arm cylinder during an operation of the swing motor. As a result, a relatively larger amount of fluid is undesirably supplied to the swing motor. In other words, the conventional variable orifice is impossible to optimally cope with a variation in load occurring at the side of the arm cylinder. Consequently, this variable orifice involves various problems such as a decrease in the operating speed of the arm cylinder, a loss of pressure and an inefficient fluid distribution.